Since CT technology can eliminate the influence of overlapped objects, it plays an important role in such occasions as security check and the like. In traditional CT, projection data on different angles are obtained by rotation of an X ray source and a detector by using a slip ring device, and a tomographic image is obtained by a reconstruction method, so as to obtain the internal information of a detected luggage. Multi-energy spectrum analysis refers to distinguishing a certain material by means of the difference of the material on absorption properties of X rays with different energy. The CT technology is cooperated with the multi-energy spectrum analysis technology to cause the existing luggage inspection equipment to reconstruct the atomic number and electron density at any position in a detected matter, in order to identify the type of the matter and play a good effect in the detection of explosives, narcotics, etc.
However, no matter the traditional CT or the multi-spectrum CT, due to the inconsistency of the X-Ray tube and the detector, reconstructed images of CT equipment with the same model always have biases on pixel values, with the increase of running hours and the ageing of the tube and the detector, even if for the same CT equipment, the reconstructed images will also drift on values and deviate from the conditions when the equipment is out of a factory. In order to solve the problems above, when the CT equipment is out of the factory and periodically maintained, calibration work will be carried out by scanning a workpiece made from a particular material, and comparing a numerical value of a reconstructed image with a true value of the workpiece to determine a post-calibrated parameter, so as to achieve a post-calibration of a CT image.
However, the above post-calibration method has the following disadvantages: on one hand, the ageing of the CT equipment is an ongoing process, while the calibration process can only be carried out periodically, for example, 1-2 times every year, which means that the post-calibrated parameter cannot be the optimal within all times, moreover, since the calibration process has strong technicality and must be achieved by a maintenance engineer, the maintenance cost is increased, and the normal working hours of the equipment may be occupied; on the other hand, due to the influence from the detector, conveyer belt, channel shape, reconstruction algorithm and the like, the difference between the pixel values and the ideal value is not the same in different areas of the CT image, while the traditional calibration method is done by placing a mid- or small-sized tool box containing the calibration workpiece on a CT conveyer belt to issue a calibration scan, thus in this method, only the image values near the conveyer belt is accurate, while an optimal correction effect cannot be achieved in other areas of the image.